Engine: Make FisherYatesShuffle actually do a Fisher-Yates shuffle

[Fayti1703]

To quote Wikipedia page on the Fisher-Yates shuffle¹:

For example, a common off-by-one error would be choosing the index j of the entry to swap […] to be always strictly less than the index i of the entry it will be swapped with. This turns the Fisher–Yates shuffle into Sattolo's algorithm, which produces only permutations consisting of a single cycle involving all elements: in particular, with this modification, no element of the array can ever end up in its original position.

rng.Next(i) only produces values up to i exclusive, so the previous code runs into precisely the situation described above.

With thanks to @samualtnorman for helping discover this.

Footnotes

1. Wikipedia contributors. Accessed 2025-02-20 UTC. ↩